The development of high-temperature superconducting (HTSC) thin-film devices for electro-optical and radio-frequency sensors is probably one of the most likely near-term outcomes of recent breakthroughs in the field of superconductivity. Among the many potential devices being considered are passive thin-film devices which can be used as detectors of electromagnetic radiation.
HTSC materials are expected to exhibit excellent performance as quantum (Josephson junction) radiation detectors at extremely high frequencies reaching into the far-infrared band; the performance of these detectors will be limited ultimately by the very high superconducting energy gap found in HTSC materials (.ltoreq.50 meV). High sensitivity is also expected for these types of detectors. Fabrication of prescribed (ideal) Josephson junctions is, however, very difficult because the junction size must be on the order of the coherence length, which is extremely short and anisotropic in these materials.
Alternatively, bolometers based on HTSC materials have been proposed because they are relatively easy to fabricate. They operate on the principle that incident radiation of virtually any wavelength will induce a resistive transition from a superconducting/state to a normal state in a thin piece of superconducting film. The substrate must be coupled to a thermal reservoir and have a low heat capacity to yield the best possible response to incident radiation in the shortest possible time. Thus, a tradeoff between response time and sensitivity in bolometric detectors exists that limits their performance relative to ideal quantum detectors.
Granular film (multiple Josephson junction) detectors, on the other hand, may be competitive as electromagnetic detectors. Granular films also appear to display nonbolometric behavior stemming, probably, from multiple weak links. A nonbolometric mechanism may be a better means of making a detector, particularly for microwave frequencies.